Metal oxide semiconductor field effect transistors (MOSFETs) are well known in the art. They operate by virtue of a field controlled channel established in a semiconductor body or surface. They come in a wide variety of forms and employ other materials besides simple metals and oxides. Persons of skill in the art understand that the word “metal” in the term MOSFET refers to any form of a electrically conductive material, as for examples and not intended to be limiting, simple metals, metal alloys, semi-metals, mixtures, semiconductors, conductive organics, conductive silicides, conductive nitrides and other conductive materials. Accordingly, the terms “metal” and “silicide” as used herein are intended to include such variations as well as other suitable conductors. A wide variety of semiconductors can be used in forming MOSFETs, such as for example and not intended to be limiting, types IV, III-V and II-VI semiconductors, organic semiconductors, and layered structures such as for example and not intended to be limiting, semiconductor-on-insulator (SOI) structures. Accordingly, the term “semiconductor” is intended to include these and other materials and arrangements suitable for forming field controlled devices. Persons of skill in the art further understand that the word “oxide” in the label MOSFET stands for any of a large number of insulating dielectrics and is not limited merely to oxides. Thus, the terms metal, oxide, semiconductor and MOSFET are intended to include these and other variations.
Further, MOSFETs can be formed with N or P type channels, depending upon the conductivity type of the various semiconductor regions and the polarity of the control voltage, and as enhancement mode or depletion mode devices depending upon the threshold voltage of the device. For convenience of explanation and not intended to be limiting, the invention is described herein for the case of N-channel devices. However, persons of skill in the art will understand that P-channel devices may be obtained by interchanging the various P and N regions of the device, that is, N-type regions are replaced by P-type regions and vice-versa. Thus, the description of N-channel devices herein serves to illustrate either N or P channel devices and the identification of particular regions of the device as being N or P conductivity type may be replaced by the more general terms “first conductivity type” or “second, opposite, conductivity type” where the “first conductivity type” may be either N or P type and the “second, opposite, conductivity type” will then be P or N type respectively, the choice depending upon what type of device (N or P Channel) is desired.
Conventional MOSFETs can inherently include parasitic bipolar devices. While such parasitic bipolar devices may not interfere significantly with operation of the MOSFET under many operating conditions, their existence can significantly degrade device properties when the device is operated at extremes of voltage and/or current. This can provide a device safe operating area (SOA) that is smaller than desired and/or the device can be more susceptible to transient stress failure than is desired. Thus, such parasitic bipolar interaction can lead to MOSFETs that are less robust than desired.
Accordingly, it is desirable to provide a new type of MOSFET with improved operating characteristics, and more particularly, MOSFETs with enhanced safe operating area (SOA), and whose parasitic bipolar operation is substantially defeated with little adverse affect on the series ON-resistance of the device. In addition, it is desirable to provide a structure and method for fabricating MOSFETs suitable for use and co-fabrication with complex devices and/or integrated circuits and especially with state of the art Smart Power technologies. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.